A standard thermostatically regulated valve such as described in German patent 3,530,812 filed Aug. 29, 1985 by J. Kostorz has hot- and cold-water inlets connected to respective valve seats whose other sides open into an outlet compartment from which tempered water flows. A double valve body is displaceable in one direction to increase the flow from one of the inlets to the outlet compartment and decrease the flow from the other inlet and is oppositely movable for the opposite effect. This valve body can be moved by axially displacing an externally displaceable stem, and is also provided with a temperature-sensitive element in the outlet compartment that can change length to move the valve element and keep it at the setting it is originally put into. Thus once a given mixed-water temperature is set, the temperature-sensitive element will automatically move the valve bodies in response, for instance, to varying supply temperatures to keep the output temperature steady.
The manual setting of such a valve is typically by means of a knob threaded on the rotatable and axially displaceable valve stem and itself constrained against axial movement, so that as it is rotated the stem is moved axially. Thus to increase the temperature the knob is rotated in one direction, thereby moving the stem axially one way, and to decrease the temperature the knob is screwed oppositely.
It is standard to provide a stop that prevents the knob from being rotated in the direction increasing temperature beyond a predetermined maximum-temperature position. This stop prevents the user from scalding himself or herself with water that is too hot. As a rule the stop is adjustable by means of a tool in order to change the maximum-temperature setting.
In some jurisdictions the maximum-temperature setting must be fixed, that is not within the control of the user, at least without using tools. The standard valve described above cannot be used in these regions.